In patients with atrial fibrillation, which heart sound would typically be absent?

In patients with atrial fibrillation, the fourth heart sound (S4) is typically absent. The presence of S4 is closely associated with a well-coordinated atrial contraction occurring just before the first heart sound (S1). In a healthy heart rhythm, the atria contract effectively to fill the ventricles, contributing to the S4 sound. However, in atrial fibrillation, the disorganized electrical activity in the atria leads to a loss of this coordinated contraction, meaning the atrial contribution to ventricular filling is significantly diminished, and thus S4 is not generated.

This absence of S4 is indicative of the underlying pathology of atrial fibrillation, where the irregular and often rapid rhythm prevents the atrial contraction necessary to produce this heart sound. Understanding the relationship between heart sounds and the cardiac mechanical events they signify is crucial for assessing patients with irregular rhythms like atrial fibrillation.